1. Field of the Invention
The invention relates generally to the field of communications. More particularly, the invention relates to AM and TV broadcast communications.
2. Discussion of the Related Art
In standard broadcast communications, numerous stations co-operate or share common broadcast channels. Typically, the Federal Communications Commission (FCC) allocates stations on a given channel in geographically separated parts of the country, so that to avoid objectionable interference, co-channel stations are typically separated by hundreds or even thousands of miles, depending on the type of station (AM or TV), the type of channel (local, regional, or long-range for AM broadcasting), and the various stations' transmitting power levels.
Perhaps the major source of interference to broadcasting stations of these types are the co-channel signals from similar stations in adjacent areas of the country. In television, the co-channel distant video carrier (which is amplitude-modulated as well) causes the generation of diagonal interference patterns (“herringbone”) in the picture, plus often a visible, horizontally oriented “sync-bar” which very slowly drifts vertically in the received display. To minimize the severity of these effects when the two carriers were very close to the same frequency, the FCC long ago mandated that geographically adjacent TV stations offset their carrier frequencies from each other by roughly 10 kHz, to keep the magnitude of the resulting interference pattern to fringe-area viewers acceptably low. However, this scheme is practical only because of the considerable width (6 MHz) of the TV channels; no such separation is feasible for amplitude-modulated (AM) radio stations, whose channel allocations are only 10 kHz in total width! Traditionally, the FCC has required that AM broadcast stations operate within ±20 Hz of their assigned carrier frequencies, which theoretically permits difference-frequency beats between co-channel stations of up to 40 Hz. Typically, the stations operate at much smaller frequency errors; beat frequencies are generally below 5 Hz, and are sometimes even below 1 Hz. Unfortunately, in areas that are located far from the local transmitter (i.e., the desired station), distant station carrier-beat components usually cause fast fluttering-type modulations of, and/or large-amplitude swishing sounds in, the desired station's audio at the receiver and concurrent distortion of the audio modulation from the distant station(s), even if they are sub-Hertz in nature, since the received desired station's carrier's amplitude and phase are noticeably modulated by the distant station's carrier signals.
AM radio receivers are virtually always provided with automatic gain-control circuitry. However, the typical AM radio receiver's automatic gain-control (AGC), also called “automatic volume control” (“AVC”), usually responds far too slowly to “average out” or suppress these beat modulations; thus, these highly annoying modulation effects are largely passed on intact to the listener.
These beat-related effects are the principal factor in the degradation of evening and nighttime AM fringe-area reception quality and the resulting loss of listeners for virtually all AM stations. As discussed above, the corresponding effect among co-channel television stations leads to beats of roughly 10 kHz rate which cause faint but often noticeable diagonal interference lines from a distant station in the picture broadcast from a more local station.
Heretofore, the requirements of reducing co-channel interference and mitigating beat frequency related effects have not been fully met. Therefore, what is needed is a solution that reduces co-channel interference and mitigates beat-related effects, preferably for both radio and television.